There are known several commercial powder metals for the fabrication of metal parts by sintering, compacting and heat treating. Many of these powder metals are used to make parts to be assembled into final machine products on high speed assembly lines. Others are used to manufacture parts that will be stored for a period of time and used upon need or demand. Often parts are fabricated for eventual assembly, that are similar in appearance, feel and composition. It is desirable that these parts have some distinguishing characteristics that will identify each part as being different from the other similarly shaped parts. Various methods of identification have been used, one of the most common being a color coding of the manufactured parts.
In the color coding of non-metallic parts, there are objections completely different from the color coding of metallic parts. In coloring plastics, for example, the pigment or dyes can be incorporated during the polymerization process and can be chemically bonded within the polymer structure itself. Treatment of these materials by colorants for identification or for decorative reasons can therefore be internal or surface treatment. Problems such as rusting, or color washing or bleeding are not encountered in this type of non-metallic system. Color treatment of this type of molded or laminated plastics is disclosed, for example, in U.S. Pat. No. 2,971,861. In this patent dye-coupling and color developing treatments are employed.
The problems in coloring metals present an entirely different set of objectives and goals. Several methods of coloring metal surfaces are disclosed in the prior art. Just as coloring of resins or plastics differ from coloring of metals, coloring of ferrous compositions differ substantially from the coloring of non-ferrous types of metal formulations. Methods of dying non-ferrous metals such as zinc and galvanized metals are disclosed in U.S. Pat. Nos. 2,393,640 and 4,314,859. In this first patent a corrosion resistant coating is deposited on a zinc and cadmium metal surface by the use of an aqueous acidic solution containing a water soluble chromium compound. The process disclosed in U.S. Pat. No. 4,314,859 also involves the use of chromate material; however, in this process the chromate dye is used in a different color intensity to form the color film. In another known process, to color treat a non-ferrous metal surface such as lead-copper, the inherently colored metal such as copper is coated over the other metal to impart a colored surface. However, in this type of system, one is limited to the color of the coating metal, the only variation being in the intensity of the color. Such a system is disclosed in U.S. Pat. No. 2,033,240.
In these above discussed prior art systems, it is not practical to form multi-colored parts because of the lack of colorants available to impart the color. This is because of the limitations on the types of electrolyzable metals available as coatings, and the limitations on colors possible, i.e., coppery or silvery colors. Also other problems can be present in these coloring systems, such as lack of color intensity, adhesion problems, or metal coating difficulties associated with the electrolyzing process. To minimize these problems, metal color coatings have been combined with chemical dyeing such as in the process described in U.S. Pat. No. 3,405,014. However, these processes have been found to be complex and sometimes difficult to control for uniform coloring.
There are also prior art processes known for the color treatment of ferrous type metallic compositions. In some methods the colorant is surface coated on the ferrous sulfate composition, resulting in a chemical coating such as a metal phthalocyanine. In many of these systems, the coated metal will lose the metallic appearance and will take on the visual appearance of the paint or colorant used. Also, the resistance of the coating to solvents or the atmosphere is often very low. In addition, the coating is generally thick and can chip off or crack when the metal piece is in use or handled. An important further drawback to these used processes is that the coating destroys or reduces the porosity of the surface, thereby making it very difficult to oil impregnate. The desired adhesion qualities of these coatings usually are lacking and their heat resistance is not what is generally desired. To solve these difficulties, various processes have been suggested wherein the ferrous metal surface is chemically converted to a metal porphyrazine or phthalocyanine. In these processes, the finished metal part is immersed in a solution containing a phthalonitrile and at a temperature below the melting point of the metallic surface. After immersion for a period of time, the surface of the metal is converted to the metal porphyrazine. The metal complex thus formed is resistant to cracking and heat, and generally cannot be destroyed. Also, the color imparted can be somewhat varied depending upon the reaction conditions used during the chemical conversion of the metal to the metal complex. Since the coating is part of the metal surface itself, the adhesion qualities are far greater than a chemical surface coating on the metallic part. This type of color coating or conversion process is disclosed in U.S. Pat. No. 2,163,768. The drawback to this prior art color method is that reaction to the desired color intensity of the metallic part is sometimes difficult to control. Also this prior art process does not lend itself to always imparting a uniform coating to the part having the same physical properties. The alteration of the physical characteristics of the part may take place when the entire part is immersed into a strongly reactive chemical solution; for example, parts that are precision fitted can be reduced in size or otherwise modified upon reaction. Furthermore, parts such as threaded screws, bolts or nuts can be adversely affected by a reactive chemical treatment.